A helicopter is generally provided with at least two turboshaft engines which operate at similar modes that depend on the flight conditions of the helicopter. Throughout the following text, a helicopter is said to be in a cruise flight situation when it is progressing in normal conditions during all the flight phases apart from transitional phases of take-off, ascent, landing or hovering flight. Throughout the following text, a helicopter is said to be in a critical flight situation when it is necessary for it to have available the total installed capacity, i.e. during the transitional phases of take-off, ascent, landing and the mode in which one of the turboshaft engines malfunctions, referred to by the abbreviation OEI (One Engine Inoperative).
It is known that, when the helicopter is in a cruise flight situation, the turboshaft engines operate at low power levels, below their maximum continuous thrust. These low power levels result in a specific consumption (hereinafter SC), which is defined as the relationship between the hourly fuel consumption by the combustion chamber of the turboshaft engine, and the mechanical power provided by said turboshaft engine, which is approximately 30% greater than the SC of the maximum take-off thrust, and thus an overconsumption of fuel during cruise flight.
Moreover, the turboshaft engines of a helicopter are designed so as to be oversized in order to be able to keep the helicopter in flight in the event of failure of one of the engines. This flight situation corresponds to the OEI mode described above. This flight situation occurs following the loss of an engine, and results in the fact that each functioning engine provides a power that is significantly greater than its rated power in order to allow the helicopter to overcome a dangerous situation, and to then continue its flight.
At the same time, the turboshaft engines are also oversized so as to be able to ensure flight in the entire flight range specified by the aircraft manufacturer, and in particular flight at high altitudes and during hot weather. These flight points, which are very restrictive, in particular when the helicopter has a mass that is close to its maximum take-off mass, are only encountered in specific use cases.
These oversized turboshaft engines have an adverse effect in terms of mass and in terms of fuel consumption. In order to reduce this consumption during cruise flight it is envisaged to stop one of the turboshaft engines during flight and to place it in a mode referred to as standby. The active engine or engines then operate at higher power levels in order to provide all the necessary power, and therefore at more favourable SC levels.
In FR1151717 and FR1359766, the applicants have proposed methods for optimising the specific consumption of the turboshaft engines of a helicopter by means of the possibility of putting at least one turboshaft engine into a stable power mode, known as continuous, and at least one turboshaft engine into a particular standby mode that it can leave in an urgent or normal manner, according to need. Leaving the standby mode is said to have occurred normally when a change in the flight situation requires the turboshaft engine in standby to be activated, for example when the helicopter is going to transition from a cruise flight situation to a landing phase. Leaving standby mode normally in this manner occurs over a period of between 10 seconds and 1 minute. Leaving the standby mode is said to have occurred urgently when a failure of or a power deficit in the active engine occurs, or when the flight conditions suddenly become difficult.
Leaving standby mode urgently in this manner occurs over a period of less than 10 seconds.
The applicants have thus proposed in particular the following two standby modes:                a standby mode known as assisted super-idling, in which the combustion chamber is ignited and the shaft of the gas generator rotates, in a mechanically assisted manner, at a speed of between 20 and 60% of the nominal speed. A mode of this kind makes it possible for the gas generator to be at the lowest possible rotational speed in order to minimise the fuel consumption. In order to improve the performance of the gas generator at this low speed, it is proposed to inject mechanical energy into the gas generator by means of an external source.        a standby mode known as banking, in which the combustion chamber is extinguished and the shaft of the gas generator rotates, in a mechanically assisted manner, at a speed of between 5 and 20% of the nominal speed. A mode of this kind makes it possible to keep a rotation of the gas generator in a speed range that permits more rapid ignition of the combustion chamber if necessary.        
These two standby modes thus require the gas generator to be continuously assisted. The duration of the assistance can be several hours during the helicopter mission. The technical problem is therefore that of providing a method for mechanically assisting a turboshaft engine in standby mode. A further technical problem is that of providing an architecture of a propulsion system that makes it possible to ensure the mechanical assistance of the gas generator of a turboshaft engine in standby mode during the mission.